1. Field of the Invention
This invention relates generally to methods of forming three-dimensional images using sonar. In particular, the present invention provides a method and apparatus for developing a statistical model of a volume of space for use in applications such as terrain mapping, terrain following and obstacle avoidance.
2. Background Information
Accurate three-dimensional representations of the underwater environment are critical for applications such as autonomous vehicle guidance, identification of underwater objects and terrain profiling. However generating such a representation is often a difficult and time-consuming process.
Existing systems use a variety of methods to achieve varying levels of approximations to the underwater environment. One such system is the sidescan sonar system. In sidescan sonar systems, arrays of elements are used to generate a beam that is narrow in the horizontal direction (approximately 1.5 degrees) but relatively wide (on the order of 50 degrees) in the vertical direction. This narrow fan beam illuminates a swath of terrain perpendicular to the direction traveled by the sonar system. Backscatter signals from the underwater structures illuminated with the beam are recorded over time and mapped to a row of pixels that represents the terrain illuminated with that insonification.
The original sidescan sonars measured backscatter intensity against time of arrival. The backscatter intensity measured as a function of time was then mapped to a grid of pixels. This mapping was performed under the assumption that the terrain was level. The level-bottom assumption can lead to the formation of artifacts in the terrain map due to acoustic shading.
An article entitled "Three-Dimensional Modeling of Seafloor Backscatter from Sidescan Sonar for Autonomous Classification and Navigation" by W. Kenneth Stewart published in The Proceedings of the 6th International Symposium on Unmanned Untethered Submersible Technology in June 1989 discusses problems such as acoustic shading. Stewart proposed the use of bathymetric data to enhance terrain maps generated by sidescan sonar systems. Recent sidescan sonars have included a second array of sonar elements parallel to the original array elements. The addition of this second array of elements permits the calculation of the angle of arrival of a backscatter signal. This angle of arrival data is used to construct a bathymetric model of the terrain illuminated by that insonification. The bathymetric model is then used to correct the terrain map.
Bathymetric corrected sidescan sonar provides a fairly accurate representation of the terrain. However it is fairly time consuming and requires at least one pass over the terrain before the data can be used for autonomous vehicle guidance.
Other approaches to three-dimensional image generation using active sonar systems are discussed in an article entitled "Three-Dimensional Map Generation From Side-Scan Images" by J. M. Cuschieri and M. Hebert published in Transactions of the ASME, Vol. 112, June 1990. Cuschieri and Hebert describe the use of forward looking systems similar to side-scan systems that are capable of mapping the area in front of an autonomous vehicle. These systems generate a beam that is similar to the side-scan beam (narrow in the horizontal direction and wide in the vertical direction). In addition, they are capable of steering the beam either mechanically or electronically to illuminate more than one vertical swath of the volume in front of the vehicle.
Forward looking sonars of this type increase in complexity with increasing horizontal resolution. The horizontal resolution of such systems is a function of the horizontal width of the beam and the number of discrete angles illuminated in the horizontal direction. A forward looking system with high resolution in both the horizontal and the vertical direction would be prohibitively expensive.
Finally, systems have been proposed that form a three-dimensional image from a single active sonar transmission. Such a system typically requires that the system insonify the entire volume of interest and then use complex beamforming techniques to sense the amplitude of the backscatter from each of the points within that volume. The resolution of such a system is directly dependent on the number of beams formed and narrowness that can be achieved for each beam.
Systems that can form a three-dimensional image from a single active sonar transmission are limited only by the speed of image processing and the number of beams formed. Since current systems are limited in the number of beams by the cost of the system and the complexity of the electronics, the result is typically a blurred image with contrast limited by the sidelobe levels achieved by the array/beamformer.
As can be seen from the above discussion, methods for generating a representation of a three-dimensional image vary in speed, effectiveness and practicality. It is clear that there has existed a long and unfilled need in the prior art for a simple, effective method of generating a three-dimensional image of a volume of water that can be used for such applications as guiding an autonomous vehicle. The present invention meets this need while avoiding these and other shortcomings of the techniques known in the prior art.